Global brain dynamics under visual entrainment

Cognitive and perceptual functions depend on the momentary dynamics of the brain. A typical approach to investigate these dynamics is through the so-called method of entrainment. In entrainment, a rhythmic stimulation (e.g., a periodic visual or auditory stimulus) leads to the mutual resonance between an external source and the activity of the brain: a light flickering at 10 Hz induces a 10 Hz oscillatory dynamic in the brain. It is often assumed that there is a one-to-one correspondence between the inducer rhythm and the process entrained in the brain. For example, performance in various cognitive and perceptual tasks is typically measured at the peaks and throughs of an induced rhythm, under the idea that the two are linked to distinct states in the brain (e.g., excitatory, or inhibitory cycles of activity). Yet, the actual effect of these periodic stimulations, which are rarely encountered in nature, remains largely unexplored. Here, we use electroencephalography (EEG) and visual entrainment to evaluate changes in global brain states and directional activity flows during rhythmic visual stimulation. In the entrainment paradigm, participants were presented with an oval flash around the center of the visual field. The luminance of the oval changed either periodically (10 Hz) or randomly for ~1.5 s. When comparing the two conditions, a strong phase synchronization at 10Hz and in the harmonics was found in the entrainment condition. In contrast, participants showed mixed effects in power. In further investigations, we will compare the temporal dynamics of micro-states observed as well as the dominant EEG topographies and the time-varying structure of lagged dependencies amongst EEG signals at different electrodes. The outcome of this work will eventually provide a more in-depth characterization of the dynamics of brain activity under entrainment, aiding the understanding of this phenomenon and the interpretation of entrainment results.